Self-driving circuit for a DC/DC converter

ABSTRACT

The present invention provides a self-driving circuit of a low voltage, large current, and high power density DC/DC converter. The converter comprises a transformer, power MOS transistors (S), output rectification portion (SRb 1, SR 2 ), filter portion and demagnetizing portion. The first configuration of the self-driving circuit consists of Da, Ra, Ca, Qa for self-driving SR 2;  and the second configuration consists of Da, Ra, Sa, a delay driving circuit and an isolation differential circuit, for self-driving SR 2.  The self-driving circuit of the present invention may reduce the cross-conductive loss, and increase the converting efficiency.

The present invention relates to a self-driving circuit for a DC/DCconverter of low voltage, high current, and high power density.

With the rapid development of high technologies, such as,communications, remote sensing, electronic computers, and electronicinstrument, the requirement of power supplies of such electronicequipment has increased accordingly. DC/DC converter of low voltage,high current, and high power density is the core technology of the powersupplies for supper integrated circuits and high-speed centralprocessors. To meet high efficiency and high power density requirement,the auxiliary (secondary) side of such DC/DC converter shall usesynchronous rectifying transistor MOSFET in place of Schottky diode forrectification in order to reduce power loss. However, for a synchronousrectifying MOSFET, the gate thereof needs a corresponding drive circuitto stimulate. In order to avoid cross-conductive losses, the requirementof timing of the drive circuit is high. The existing drive circuitsutilize external driving technology, but its control is too complicatedand the cost is high.

FIG. 1 a shows a demagnetized forward stimulating circuit of threewindings. The voltage waveform of its secondary side is shown in FIG. 1b. FIG. 2 is a self-driving circuit of the synchronous rectifying MOSFETtransistors SR₁ and SR₂ of the circuit of FIG. 1 that are driven by thevoltage waveform of the secondary side. One of the synchronousrectifying transistors SR₁ (for rectification) may be directly driven bythe voltage waveform of the secondary side of the transformer, while theother synchronous rectifying transistor SR₂ (for rectification) cannotbe directly driven by the voltage waveform of the secondary side of thetransformer. This is because there is a dead interval from thedemagnetizing stage to simulating stage in the voltage waveform of thesecondary side of the transformer. The existence of this dead intervalcauses the turn-off of the continuous synchronous rectifying transistor(SR₂) for lack of drive during this dead interval. However, in order tomaintain the continuity of the inductive current, the body diode of SR₂will become conductive. The characteristics of voltage drop andswitching are all poor due to the conduction of the body diode. This mayresult in increase of power loss and backward recovery loss causedthereby such that the efficiency of the entire transformer decreasesdramatically. Therefore, such a main circuit cannot use the directself-driving technology.

At present, there are two self-driving circuits that can drive the SR₂of FIG. 1 a, respectively shown in FIGS. 3 a and 3 b. In FIG. 3 a, adiode Da and a small power MOSFET Sa are used to drive the continuouscurrent SR₂. It operates as: when the secondary side voltage becomesnegative at top and positive at the bottom, SR₁ turns off, and SR₂ turnson due to the diode Da. During the dead interval t_(dead), the secondaryside voltage becomes zero, Da turns off, and SR₂ still turns on becauseits Vgs does not have any discharging path. When the secondary sidevoltage becomes positive at top and negative at bottom once again, SR₁and Sa turn on so as to turn off SR₂. This circuit can realize SR₂self-driving by using simply two components, but cannot realize the timesequence that SR₂ turns off first and SR₁ turns off later. Thus, itstill has relatively large cross-conductive loss, and may damage SR₁ andSR₂ if serious. Then the operation of the converter is not reliable. Thecircuit of FIG. 3 b is driven by PWM drive signal of the primary side.After the isolation of the pulse transformer, it drives, respectively,SR₁ and Sa at the same phase. Although it realizes SR₂ self-driving andensures that Sa turns on earlier than SR₁, thereby reducing thecross-conducting time of SR₂ and SR₁. However, it does not eliminatecompletely the cross conductive loss, and the efficiency of thetransformer is still hard to increase.

Therefore, the object of the present invention is to solve the existingproblem of self-driving technology of the commonly used main circuit ofthe DC/DC converter of low voltage and high current, and to provide aself-driving circuit that enables minimizing the cross-conductive lossand backward recovery loss, simplifying the structure, and reducing thecost.

The present invention is realized by the following technical embodiment.In the self-driving circuit of the DC/DC converter of the presentinvention, the rectification portion of the converter comprisessynchronous rectifying MOS transistors (SR₁) and (SR₂), wherein theself-driving circuit is composed of a resister (Ra), a capacitor (Ca), atransistor (Qa) and the diode (Da). The resister (Ra) and the capacitor(Ca) are connected in parallel. An end of the parallel connection isconnected with the positive end of the winding (Ns), and the other endthereof is connected with the base of the transistor (Qa). The emitterof the transistor (Qa) is connected with the source of the MOStransistor (SR₁), while its collector is connected with the cathode ofthe diode (Da) and the gate of the MOS transistor (SR₂). The anode ofthe diode (Da) is connected with the negative terminal of the winding(Ns), and its cathode is connected with the gate of the MOS transistor(SR₂).

In the other self-driving circuit of the DC/DC converter of the presentinvention, the rectification portion of the converter comprisessynchronous rectifying MOS transistors (SR₁) and (SR₂), wherein theself-driving circuit is composed of a resister (Ra), a diode (Da), asmall power MOS transistor (Sa), a time delay driving circuit, and anisolating differential circuit. The anode of the diode Da is connectedwith the negative end of the winding Ns and the drain of the synchronoustransistor (SR₁), and its cathode is connected to the gate of thesynchronous rectifying MOS transistor (SR₁) and the drain of the smallpower transistor Sa through the resistor (Ra). An end of the delaydriving circuit is connected with an end of the isolating differentialcircuit, and the other end of the isolating differential circuit isconnected with the gate of the small power transistor Sa.

The isolating differential circuit may be composed of the resistance ofthe windings Npa and Nsa of the transformer, two capacitors, tworesisters, and a diode. The resistance of the winding Nsa is connectedwith the parallel-connected resisters and diode through the capacitors.

The time delay driving circuit is composed of a delay circuit and adriving circuit, wherein an example of the delay circuit is formed byconnecting the diode and resister in parallel, and then connected to aground capacitor in serial.

The MOS transistor (SR₂) turns off before the resister (SR₁) turns on.

The DC/DC converter is a demagnetized forward converter of threewindings (Nc, Np, Ns). The negative end of the winding Np is connectedwith the drain of the power MOS transistor (S). An end of the delaydriving circuit is connected with an end of the isolating differentialcircuit, and the other end is connected to the gate of the power MOStransistor (S).

The DC/DC converter is a forward converter of resonant clamping. Thewinding Np and the capacitor (Cc) are connected in parallel, and the nconnected with the drain end of the power MOS transistor (S). An end ofthe delay driving circuit is connected with an end of the isolatingdifferential circuit, and the other end is connected to the gate of thepower MOS transistor (S).

The DC/DC converter is a double forward converter of diode clamping. Thepositive end of the winding (Np) is connected with the source of thepower MOS transistor (S₁), and the negative end of the winding (Np) isconnected with the drain of the power MOS transistor (S₂). The anode ofthe diode (D) is connected with the negative end of the winding (Np),and the cathode is connected with the drain end of the power MOStransistor (S₁). The cathode of the diode D₂ is connected with thepositive end of the winding (Np), and the anode is connected with thesource end of the power MOS transistor (S₂). An end of the delay drivingcircuit is connected with an end of the isolating differential circuit,and the other end is connected, respectively, with the gate of the powerMOS transistor (S₁) and the gate of the power MOS transistor (S₂).

The DC/DC converter is a double forward converter of resonance clamping.The positive end of the winding (Np) is connected with the source of thepower MOS transistor (S₁), and the negative end of the winding (Np) isconnected with the drain end of the power MOS transistor (S₂). Thecapacitor (Cc) and the winding (Np) are connected in parallel; the twoends thereof are, respectively, connected with positive and negativeends of the winding (Np). An end of the time delay driving circuit isconnected with an end of the isolating differential circuit, and theother end is connected, respectively, with the gates of the power MOStransistors (S₁), (S₂).

The advantages of the present invention are as follows: for the firstcircuit, it ensures the conduction of the continuous current SR duringthe dead interval through the addition of the accelerate circuit andcareful selection of Ra, Ca, Qa, and the resistance of the resistersconnected in serial at its gate, and at the same time, it maintains theminimum cross-conductive loss and high converting efficiency.

The following embodiments will be discussed in conjunction with theaccompanying drawings.

FIG. 1 a is a normal demagnetized forward converter circuit of threewindings;

FIG. 1 b is the waveform of the secondary side voltage of the abovecircuit;

FIG. 2 is a direct self-driving circuit;

FIG. 3 a is a known self-driving circuit;

FIG. 3 b is another known self-driving circuit;

FIG. 4 a is the first self-driving circuit of the present invention;

FIG. 4 b is the second self-driving circuit of the present invention;

FIG. 5 a is an embodiment of a resonant clamping forward converter usingthe first self-driving circuit;

FIG. 5 b is an embodiment of a resonant clamping forward converter usingthe second self-driving circuit;

FIG. 6 a is an embodiment of a diode clamping double forward converterusing the first self-driving circuit;

FIG. 6 b is an embodiment of a diode clamping double forward converterusing the second self-driving circuit;

FIG. 7 a is an embodiment of a resonant clamping double forwardconverter using the first self-driving circuit;

FIG. 7 b is an embodiment of a resonant clamping double forwardconverter using the second self-driving circuit;

FIG. 8 a shows a typical waveform of the converter using the converterof FIG. 4(a); and

FIG. 8 b shows a typical waveform of the converter using the converterof FIG. 4(b).

FIG. 4(a) illustrates the first embodiment of self-driving circuit ofthe present invention used in the demagnetized forward stimulatingcircuit of three windings. It is a non-isolating self-driving circuit,and is a drive circuit for SR₂, that is composed of a diode Da, aresister Ra, a capacitor Ca, transistor Qa. The circuit winding of thetransformer Np and Ns are connected through the like ends, and therectifying MOS transistor is composed of SR₁ and SR₂.

In the embodiment of the first circuit, when t=t₁, the main switch Sturns off, Dc, and generates a value of demagnetizing voltage-secondwith the auxiliary winding Nc. The waveform of the secondary sidevoltage changes from positive to negative, Vgs_(R1) decreases. After itsvoltage passes the zero, SR₁ turns off, and V_(gSR2) increases becauseof the turning-on of Da. Before reaching the starting voltage, the bodydiode will turn on first. At t=t₂, the transformer has been completelydemagnetized, the voltage of the secondary side reduces to zero, and thediode Da turns off. Thus, the gate voltage V_(gSR2) of SR₂ maintains atVin/N because there is no discharging path so that SR₂ sustainscontinuous current. When t=Ts, the main switch S turns on again, thesecondary side voltage changes from zero to Vin/N. This causes thetransistor Qa to turn on faster than SR₁ due to the accelerate circuitformed of Ra, Ca so as to ensure the rapid discharge of the gate voltageof SR₂. Through careful selection of Ra, Ca, Qa and the resistance ofthe serial connected resisters at its gate, it may reduce dramaticallythe cross-conductive loss, and increase the efficiency of the converter.

Due to the storage time of the Qa when turning off, the parameters of Raand Ca can hardly properly determined when the frequency is relativelyhigh. Accordingly, for this technology, the switching frequency ispreferably lower than 250 KHz.

FIG. 4 b illustrates the second embodiment circuit of the presentinvention used in a demagnetized forward stimulating circuit of threewindings. In this embodiment, it utilizes a small power MOSFET, a set ofan isolating differential circuit and a time delay circuit to accomplishthe function of Ra, Ca, Qa of the first circuit. The structure is asfollows: the anode of the diode Da is connected to the negative end ofthe winding Ns and the drain end of the synchronous rectifying MOStransistor (SR₁), and the cathode is connected, through the resister(Ra), with the gate of the synchronous rectifying MOS transistor (SR₁)and the drain end of the small power transistor Sa. An end of the delaydriving circuit is connected with an end of the isolating differentialcircuit, and the other end is connected to the gate of the small powertransistor Sa. Turning-on of SR₂ is the same as that in FIG. 4 a, whileits turning-off is realized by the delay circuit and the isolatingdifferential circuit.

The isolating differential circuit may be composed of the windings Npaand Nsa of the transformer, two capacitors, two resisters and a diode.The winding Nsa is connected, through the capacitors, to theparallel-connected resisters and diode.

The time delay driving circuit is constructed by a delay circuit and adriving circuit. As an example of the delay circuit, the diode and theresister can be connected parallel, and then connected in serial withthe ground capacitor.

The function of the delay circuit is to enable that SR₂ turns off beforeSR₁ turns on, and to control the optimum time delay so as to maximizethe efficiency of the converter. This converter does not have anyrestriction to the frequency of switch. The size of all of thecomponents is relatively small.

The two self-driving circuits of the present invention disclosed hereinhave been proved through experiments. The first circuit has been used ina DC/DC power supply (using resonant clamping forward circuit) of directcurrent input of 40-60 V and direct current output of 2.5V/50V. Theefficiency of the power stage reaches over 90%. The second circuit hasbeen used in a DC/DC power supply (using resonant clamping forwardconverting circuit) of direct current input of 37-72 V and directcurrent output of 5V/30V. The efficiency of the power stage exceeds 90%.

FIG. 5 to FIG. 7 illustrate the various application circuits using thesetwo circuit embodiments.

FIG. 5 shows that the two self-driving circuits are used in theresonance clamping forward converter. The DC/DC converter is resonantclamping forward converter. The winding (Np) and the capacitor (Cc) areconnected in parallel, and then connected to the drain end of the powerMOS transistor.

FIG. 6 shows the application of the two self-driving circuits of thepresent invention in the diode clamping double forward stimulatingcircuit. The positive end of the winding (Np) is connected with thesource of the power MOS transistor (S₁), and the negative end of thewinding (Np) is connected to the drain of the power MOS transistor (S₂).The anode of the diode (D₁) is connected with the negative end of thewinding (Np), and the cathode is connected with the drain end of thepower MOS transistor (S₁). The anode of the diode (D₂) is connected withsource of the power MOS transistor (S₂), and the cathode is connectedwith the positive end of the winding (Np).

FIG. 7 is the application of the two self-driving circuits of thepresent invention in the resonant clamping double forward stimulatingcircuit. The positive end of the winding (Np) is connected with thesource of the power MOS transistor (S₁), and the negative end of thewinding (Np) is connected to the drain of the power MOS transistor (S₂).The capacitor (Cc) is connected in parallel with the winding (Np), itstwo ends are connected, respectively, with the positive and negativeends of the winding (Np).

In the various applications in FIGS. 5-7, the function of the twoself-driving circuits of the present invention is to minimize the crossloss of the MOS transistors SR₁ and SR₂, and maximize the convertingefficiency.

The present invention has been explained above through the embodiments.However, the present invention is not limited thereto. Any improvementand substitution should be viewed within the scope of the protection ofthe present invention provided that they are not apart from the spiritsand contents of the present invention.

1. A self-driving circuit of a DC/DC converter, said converter includingat least a transformer having a primary winding and a secondary winding,and a rectification portion having at least a first and a second ofsynchronous rectifying MOS transistors, characterized in that saidself-driving circuit comprises a resister, a capacitor, a transistor,and a diode, said resister and capacitor being connected in parallel,one of the ends of said parallel connection being connected with thepositive end of the secondary winding, and the other connected to thebase of said transistor; the emitter of said transistor being connectedto the source end of the first synchronous rectifying MOS transistor,and the collector of said transistor connected with the cathode of saiddiode and the gate end of the second synchronous rectifying MOStransistor; and the anode of said diode being connected to the negativeend of the secondary winding.
 2. The self-driving circuit of claim 1,characterized in that said second synchronous rectifying MOS transistorturns off before said first synchronous rectifying MOS transistor turnson.
 3. The self-driving circuit of claim 1, characterized in that saidDC/DC converter is a demagnetized forward converter, which includes athird winding and a power MOS transistor in addition to said primary andsecondary windings, the negative end of said primary winding isconnected with the drain end of said power MOS transistor.
 4. Theself-driving circuit of claim 1, characterized in that said DC/DCconverter is a resonant clamping forward converter, the positive end ofthe primary winding being connected parallel with a capacitor and thenconnected to the source end of a power MOS transistor.
 5. Theself-driving circuit of claim 1, characterized in that said converter isa diode clamping double forward converter, the positive end of theprimary winding being connected with the source end of a first power MOStransistor, and the negative end of the primary winding connected withthe drain end of a second power MOS transistor, said converter furtherincluding a first and a second diode, the anode of said first diodeconnected with the negative end of said primary winding, and the cathodeconnected with the drain end of said first power MOS transistor, theanode of said second diode being connected with the source end of saidsecond power MOS transistor, and the cathode connected the positive endof said primary winding.
 6. The self-driving circuit of claim 1,characterized in that said converter is a resonant clamping doubleforward converter, the positive end of the primary winding beingconnected to the drain end of a first MOS power transistor, and thenegative end of the primary winding connected to the drain end of asecond power MOS transistor, said resonant clamping double forwardconverter further including a capacitor connected in parallel with saidprimary winding.
 7. A self-driving circuit of DC/DC converter, saidconverter including at least a transformer having a primary winding anda secondary winding and a rectification portion having at least a firstand a second synchronous rectifying MOS transistors, characterized inthat said self-driving circuit includes a first resister, a first diode,a small power MOS transistor, a delay driving circuit and an isolatingdifferential circuit, the anode of said first diode being connected withthe negative end of the secondary winding and the drain end of saidfirst synchronous rectifying MOS transistor, and the cathode of saidfirst diode connected, through said resister, with the gate end of saidsecond synchronous rectifying MOS transistor and the drain end of saidsmall power MOS transistor; one of two ends of said delay drivingcircuit connected with one of two ends of said isolating differentialcircuit; and the other end of said isolating differential circuitconnected with the gate end of said small power MOS transistor.
 8. Theself-driving circuit of claim 7, characterized in that said isolatingdifferential circuit includes a first winding and a second winding and asecond winding of a second transformer, a first and a second capacitor,a second and a third resister and a second diode, said first capacitorbeing connected with said first winding, said second resister and saidsecond diode connected in parallel, and said second winding connected,through said second capacitor, in parallel with said parallel connectedsecond diode and second resister.
 9. The self-driving circuit of claim7, characterized in that said delay driving circuit includes a delaycircuit and a driving circuit, wherein said delay circuit is comprisedof a diode and a resister parallel connected, and a ground capacitorconnected in serial.
 10. The self-driving circuit of claim 7,characterized in that said second synchronous rectifying MOS transistorturns off before said first synchronous rectifying MOS transistor turnson.
 11. The self-driving circuit of claim 7, characterized in that saidDC/DC converter is a demagnetized forward converter, further including athird winding and a power MOS transistor in addition to said primarywinding and secondary winding, the negative end of said primary windingbeing connected with the drain end of said power MOS transistor, an endof said delay driving circuit connected with an end of said isolatingdifferential circuit, and the other end connected to the gate end ofsaid power MOS transistor.
 12. The self-driving circuit of claim 7,characterized in that said DC/DC converter is a resonant clampingforward converter, the primary winding being connected parallel with acapacitor, and then connected to the source of a power MOS transistor,an end of said delay driving circuit being connected with an end of saidisolating differential circuit, and the other end connected with thegate end of said power MOS transistor.
 13. The self-driving circuit ofclaim 7, characterized in that said DC/DC converter is a diode clampingdouble forward converter, the positive end of the primary winding beingconnected with the source of a first power MOS transistor, and thenegative end of the primary winding connected with the drain end of asecond power MOS transistor, said converter further including a firstand a second diode, the anode of said first diode being connected withthe negative end of said primary winding, and the cathode connected withthe drain end of said first power MOS transistor, the anode of saidsecond diode being connected with the source end of said second powerMOS transistor, and the cathode connected with the positive end of saidprimary winding, said delay driving circuit connected with an end of theisolating differential circuit, and the other end connected with thegate end of said first and second power MOS transistor, respectively.14. The self-driving circuit of claim 7, characterized in that saidDC/DC converter is a resonant clamping forward converter, the positiveend of the primary winding being connected with the source end of afirst power MOS transistor, and the negative end of the primary windingconnected with the drain end of a second power MOS transistor, saidconverter further including a capacitor connected parallel with saidprimary winding, and the two ends of the parallel connection connected,respectively, with the positive and negative ends of said primarywinding, an end of said delay driving circuit being connected with anend of the isolated differential circuit, and the other end connected,respectively, with the gate ends of said first and second power MOStransistors.